Recently, uses of thermoplastic elastomer composition are being steadily increased including parts of automobiles, household appliances, etc. because of their low hardness and flexible properties and their moldability by the methods such as extrusion molding, injection molding, etc. like the conventional thermoplastic resins.
Especially, olefinic thermoplastic elastomers are superior in flexibility, heat resistance and low temperature impact resistance and hence are in increased demand as exterior trim parts due to the trend of lightening of automobiles, elaboration in design of automobiles and industrial rationalization.
Typical example thereof is bumper part, which is a large functional part and is required to have superior injection moldability, heat resistance, mechanical properties such as low temperature impact resistance, etc., superior appearance of molded products and superior paintability.
This invention relates to a thermoplastic elastomer composition capable of providing large molded products good in injection moldability and excellent in appearance with keeping low temperature impact resistance and a process for producing such composition. This composition is especially intended to use as the automobile parts as mentioned above.
Hitherto, compositions comprising olefinic copolymer rubbers and olefinic plastics have been generally used as olefinic thermoplastic elastomers. However, in order to effectively exhibit the characteristics such as heat resistance and low temperature impact resistance, partial crosslinking of olefinic copolymer rubbers with organic peroxides or the like has been generally carried out.
Under the ordinary temperature and/or pressure conditions for injection molding of thermoplastic elastomers, viscosity of olefinic copolymer rubber used as a component of the elastomer is high and besides, partial crosslinking thereof with organic peroxide causes further decrease of melt-flowability of the thermoplastic elastomer. Therefore, when the thermoplastic elastomer is injection molded, if the article to be produced is of large size and of thin-wall, molten thermoplastic elastomer in mold changes greatly in its melt-flowability and is difficult to fill uniformly into small parts of the mold or undesirable phenomena such as flow marks or weld lines occur on the surface of molded articles, which considerably damage commercial value of the articles.
Especially, for exterior trim parts of large size represented by bumper parts, high melt flowability at injection molding and excellent appearance of molded articles are essential together with flexibility, heat resistance, low temperature impact resistance and paintability and thermoplastic elastomers having all of these properties have been demanded.
Various improvements have been proposed in an attempt to attain these objects.
For example, Japanese Pat. Publication (Kokoku) No. 15741/81 discloses that injection molded products of large size and of thick-wall having surface appearance with less sink marks and flow marks can be obtained with addition of peroxide non-crosslinking type hydrocarbon rubber materials and/or mineral oil type softening agents.
Furthermore, Japanese Pat. Publication (Laid-open) (Kokai) No. 34837/83 proposes a method for improving degree of flow marks formed on the surface of injection molded articles by heat treating a composition comprising a specific olefinic copolymer rubber and a specific olefinic plastic in the presence of an organic peroxide and a bismaleimide compound and then homogeneously blending a specific olefinic plastic with said composition.
However, these methods cannot still provide tensile strength and tesile elongation sufficient to develop low temperature impact resistance and furthermore, there often occurs difference in surface gloss between the end position and the position near the gate in an injection molded article of large size and besides, considerable flow marks often occur. Thus, satisfactory results have not yet been obtained.